SAW components are assembled on piezoelectric substrates, mono-crystalline wafers being preferred, due to their favorable piezoelectric characteristics. The piezoelectric characteristics as well as a number of other characteristics, such as the propagation velocity of acoustic waves in the wafer, is dependent upon the orientation of the wafer surface relative to the crystal axes of the piezoelectric mono-crystal. Through suitable choice of the crystal cut, wafers can be made available in this manner whose cut-dependent characteristics support the desired performance of the SAW component.
Wafers with cut angles that support the effective generation and low-loss propagation of surface-proximate acoustic waves are normally used for SAW components. These include, for example, quartz wafers with an ST cut, lithium niobate wafers with a rot YX cut of ca. 40-65°, and lithium tantalite with a cut angle of intersection rot YX of 36 to 46°. In the case of most components on substrates having the cut angles indicated, a wave propagating into the bulk of the substrate is normally generated in addition to the SAW. Because the acoustic energy of such a wave cannot be used in the component, this results in transfer losses. For this reason, measures are necessary to minimize these losses. So far, however, the complete suppression of leakage wave losses has not been possible.
Another problem with substrates suitable for SAW components lies in the relatively high temperature coefficient of frequency. This refers to the temperature dependency of substrate characteristics, such as the propagation velocity of the surface wave. Ultimately, this also causes a temperature dependency of the center frequency of the component. Leakage wave substrates, when compared with quartz, exhibit a relatively high temperature coefficient of frequency TCF of ca. 40 ppm/K. To absorb this temperature coefficient of frequency, the bandwidth of SAW components produced thereon must be increased sufficiently so that the component and, in particular, an SAW filter can still fulfill the required specification.
The duplexer required for the US-PCS mobile wireless system is a filter application whose specifications place high demands on a component. Its specifications cannot be maintained with SAW components and/or substrate materials with the aforementioned high temperature coefficient of frequency. To this end, it would be necessary to reduce the temperature coefficient of frequency.
Various methods have already been proposed for reducing the temperature coefficient of frequency, each of which, however, is associated with another serious disadvantage.
Reversing the piezoelectric axis of the piezoelectric substrate on the surface of the wafer, thereby reducing the temperature coefficient of frequency, is known, for example, from an article by K. Nakamura and A. Tourlog, ‘Effect of a ferroelectric inversion layer on the temperature characteristics of SH-type surface acoustic waves on 36°Y-X LiTaO3 substrates,’ IEEE Trans. Ferroel. Freq. Ctrl. Vol. 41, No. 6, November 1994, pp. 872-875. The problem in this case, however, is the associated reduction in coupling, the difficulty in manufacturing and the limited reduction in the TCF to ca. 15 ppm/K.
Generating a thin lithium tantalate film on a wafer with lower temperature propagation is known from an article by K. Eda et al., ‘Direct Bonding of piezoelectric materials and its applications,’ IEEE Ultrason. Symp. Proc. 200, pp. 299-309. Because of the thermal distortion with the wafer, a component assembled thereon has a reduced temperature coefficient of frequency. A disadvantage worth noting in this context is that producing these substrate materials requires a complex technology, which generates high process complexity and, therefore, high costs.
Reducing the temperature coefficient of frequency of SAW components with an SiO2 film applied onto the entire surface of the substrate and the metallization is known from an article by K. Asai, M. Hikita et al., ‘Experimental and theoretical investigation for temperature characteristics and propagation losses of SAWs on SiO2/Al/LiTaO3,’ IEEE Ultrason. Symp. 2002 (to be published). In this context, however, it has come to light that the level of metallization must be substantially reduced in comparison to conventional SAW components. This results in increased attenuation, because the finger resistance in the transducers increases with reduced layer thickness. In addition, this method for reducing the temperature coefficient of frequency requires a very high layer thickness of the SiO2 film of approx. 20% h/λ (that is, relative to the wavelength of the SAW that can be propagated therein). For this reason, the quality of the SiO2 layer is crucial to the extent of the reduction achieved in the temperature coefficient of frequency and the insertion loss to be accepted.
However, implementing a US-PCS duplexer as an SAW component is not easily achieved with any of the methods proposed here.